In view of protecting the environment, fuel vapor escaping from the fuel tank has been controlled as well as the exhaust emission control. According to the regulation established by the Environmental Protection Agency (EPA) and the California Air Resourced Board (CARB), a leak detection of the fuel vapor flowing out through a small aperture of the fuel tank is strictly required.
A conventional leak check system shown in JP-10-90107A, which is a counterpart of U.S. Pat. No. 5,890,474, has a pump which generate a pressure gradient between an inside and an outside of a fuel tank. When the fuel vapor leaks from the fuel tank, a load of a motor driving the pump fluctuates. The detection of fuel vapor leakage is conducted by checking the fluctuation of the motor load.
For generating the pressure gradient between the inside and the outside of the fuel tank, the interior of the fuel tank must be pressurized or depressurized by the pump. When the pump pressurizes the interior of the fuel tank, if an opening or a hole causing leakage of fuel vapor exists at the fuel tank, the fuel vapor is expelled to the atmosphere through the opening or the hole. To prevent the leakage of the fuel vapor into the atmosphere, the interior of the fuel tank must be depressurized.
The motor has a driving shaft for driving the pump, the driving shaft being supported by a bearing. A lubricant is provided between the driving shaft and the bearing to obtain a smooth rotation of the driving shaft.
When the interior of the fuel tank is depressurized, the inside pressure of the pump decreases and the inside pressure of the motor is kept atmospheric pressure. Thus, the lubricant is introduced into the pump due to the pressure gradient, so that the lubricant adheres to the inner surface of the motor to cause a deterioration of the motor performance.
On the other hand, when the interior of the fuel tank is pressurized, the lubricant is introduced into the motor to deteriorate the motor performance.